Berkshire Hathaway has done well in 2013, but it doesn't mean a little second-guessing isn't in order

Sell No. 2: ConocoPhillips

The big news in Berkshire Hathaway’s 13F filing was its 40.1 million shares in Exxon Mobil (XOM), an addition that makes the world’s largest oil company Berkshire’s seventh-largest holding at 3.75% of its portfolio.

This is the second go-round for Buffett with XOM; he owned its stock for less than two years back in 1985, selling out for a 44% gain.

However, with XOM representing a top 10 holding, its other big E&P play — ConocoPhillips (COP) — becomes redundant.

That’s a reason for Buffett to sell. A reason for you to sell is that COP has had a much better run in recent years, and reversion to the mean suggests it and XOM are about to change directions.